When a witness knows who the primary suspect is in a lineup with their sole job being to confirm whether or not this is the correct perpetrator, they are more likely to naturally convince themselves mentally that this is the correct person and make the guilty judgment.  In fact, any form of suggestibility that may single a specific person out during identification makes them much more likely to be chosen by the witness.  When the administrator of the identification knows who the suspect is, they are likely to knowingly or even unknowingly give suggestions and hints that lead the witness into choosing the person that is already suspected.  In order to eliminate this possible contributing factor to unreliable or incorrect identification, double blind procedures make it so that the primary suspect is unknown to both the witness and the administrator.